paleomagnetisme

Paleomagnetism, or paleomagnetisme in some languages, is the study of the record of the Earth's magnetic field preserved in rocks, sediments, and archeological materials. The central idea is that magnetic minerals lock in the direction and intensity of the geomagnetic field when they form or undergo chemical change, providing a snapshot of the field at that time.

Natural remanent magnetization arises mainly in three forms: thermoremanent magnetization (TRM) in cooling igneous rocks; detrital remanent magnetization (DRM) in clastic sediments carried by grains aligned with the field during deposition; and chemical remanent magnetization (CRM) from post-depositional chemical changes. These signals can be altered by later heating, metamorphism, or chemical processes.

Scientists measure paleomagnetism by recording the magnetization of samples with sensitive magnetometers and separating ancient signals from later overprints using demagnetization procedures, such as progressive thermal or alternating-field demagnetization. The resulting directions and, when possible, the intensity of the ancient field are used in paleomagnetic dating and reconstruction.

Paleomagnetism provides information on past plate motions, continental positions, and the history of the geomagnetic field, including reversals and excursions. The polarity timescale, built from many rock records, links magnetic reversals to stratigraphy and helps date rocks and sediments. Notable features include reversals such as the Brunhes–Matuyama boundary.

Limitations include alteration after magnetization, secondary magnetic components, and anisotropy of magnetic minerals, which can complicate interpretation. Despite challenges, paleomagnetism remains a core method in geophysics and geology for understanding Earth's magnetic history and the dynamics of plate tectonics.